The invention relates to turbine engines, in particular to aviation turboshaft engines or to industrial turbines, and more particularly to a turbine nozzle stage made of ceramic matrix composite material or of composite material having a matrix that is made at least in part of ceramic, and referred to below as CMC material.
The field of application of the invention is in particular that of gas turbine aero engines. Nevertheless, the invention is applicable to other turbine engines, for example industrial turbines.
Improving the performance of turbine engines and reducing their polluting emissions leads to envisaging ever higher operating temperatures.
For elements of hot portions of turbine engines, proposals have thus been made to use ceramic matrix composite materials, written CMC materials below.
CMC materials are typically formed by fiber reinforcement made of refractory fibers, such as carbon fibers or ceramic fibers, and densified by a matrix that is made of ceramic or at least partially of ceramic.
These materials possess remarkable thermostructural properties, i.e. mechanical properties that make them suitable for constituting structural elements, together with the ability of conserving these properties at high temperatures. Furthermore, CMC materials are of density that is much lower than the density of the metal materials conventionally used for elements of hot portions of turbine engines.
Thus, Documents WO 2010/061140, WO 2010/116066, and WO 2011/080443 describe making blades of turbine engine rotor wheels with integrated inner and outer platforms out of CMC. The use of CMC materials for turbine nozzles has also been proposed, in particular in Documents WO 2010/146288, FR 2 979 662, and EP 2 443 318.
A conventional metal turbine nozzle stage is in the form of a ring made up of a plurality of assembled-together sectors, each sector having an inner platform, an outer platform, and a plurality of vanes extending between the inner and outer platforms and secured thereto. The juxtaposed inner platforms form an inner shroud and the juxtaposed outer platforms form an outer shroud. The inner and outer shrouds define a gas flow passage through the nozzle stage.
Introducing a nozzle stage, e.g. a high-pressure nozzle stage, that is made of CMC makes it possible to increase the maximum temperature that can be tolerated compared with a nozzle stage that is made of metal, thereby reducing the quantity of cooling air that needs to be used. This thus makes it possible to increase the performance of the turbine engine.
Nevertheless, because of its properties that are very different from those of metal, CMC is more sensitive to certain mechanical stresses. Specifically, CMC presents greater stiffness and smaller expansion. It behaves better in compression, but its acceptable traction stresses are smaller than those of metal.
Furthermore, it is difficult to introduce a CMC part in a metal environment because of differential thermal expansion between CMC and metal. This is particularly difficult in a turbine engine, and more particularly in a high-pressure portion of the engine, since the environment is very hot, thereby exacerbating the differences in the coefficients of thermal expansion between the materials, and the aerodynamic forces to which a high-pressure nozzle stage is subjected are also very high in this region of the turbine.
A high-pressure nozzle stage is known that comprises annular sectors that are held in position firstly by using two pegs passing through a round hole and an oblong hole made in a bottom portion of the inner platform, the bottom portion extending along a radial direction from an inside face of the bottom shroud formed by the nozzle stage, and secondly by using an abutment mounted on an outside face of the outer platform, the top portion projecting from the outer platform in the radial direction of the annular ring.
That architecture leads to an integration constraint since it is not possible to attach the annular sectors from above, i.e. to attach them to the outer casing.
The vane of a nozzle is subjected to large forces from the combustion gas. When the nozzle is attached from above, i.e. via the casing, or from below, the combustion gas causes the vane to bend, thereby constituting a source of stresses.
Documents US 2008/279679 and EP 3 009 601 disclose turbines having a nozzle stage with annular sectors having vanes with struts coupled to the casing passing therethrough.